Why Cyclohexatriene (C<SUB>6</SUB>H<SUB>6</SUB>, p<i>K</i><SUB>a</SUB> = 43) is Less Acidic than Cyclopentadiene (C<SUB>5</SUB>H<SUB>6</SUB>, p<i>K</i><SUB>a</SUB> = 15) and Cycloheptatriene (C<SUB>7</SUB>H<SUB>8</SUB>, p<i>K</i><SUB>a</SUB> = 36): A Freshmen Chemical Education Undergraduate Exercise

The pKa of Cyclohexatriene (Benzene) is 43. And that of Cyclopentadiene and Cycloheptatriene are 15 and 36 respectively. In the ascending order of number of carbon atoms of the three cyclic hydrocarbons, Cyclohexatriene lies between Cyclopentadiene and Cycloheptatriene. It is a tempting belief of undergraduate chemists who begin to pursue their undergraduate course that the pKas will be in the same increasing order with increase in ring size. But surprisingly pKa of Cyclohexatriene is more than that of either Cyclopentadiene and or Cycloheptatriene. Suitable explanations are given.